1. Field of the Invention
This invention relates to polyamides. More specifically, this invention relates to an improved process for manufacturing polyamides from an .alpha.,.omega.-dinitrile, and an .alpha.,.omega.-diamine in water in contact with a catalyst. The instant invention is a two-step polymerization process wherein the dinitrile and water react in the presence of a catalyst followed by the addition of and polymerization with the diamine. For example, the instant invention relates to an improved process for manufacturing nylon-6,6 from adiponitrile and hexamethylenediamine in water in contact with an oxygenated compound of phosphorus as a catalyst, utilizing this two-step polymerization process.
2. Description of the Prior Art
At the present the commercial route to nylon-6,6 is the polymerization of a dicarboxylic acid (i.e. adipic acid) and a diamine (i.e. hexamethylenediamine). Alternative routes to nylon-6,6, such as by the polymerization of a dinitrile and a diamine, have long been recognized. For example, U.S. Pat. No. 2,245,129, issued in 1938 to Greenwalt, describes a method for producing a linear polyamide by heating the reaction mixture of a dinitrile, a diamine and water. The Greenwalt procedure consists of two stages. In the first stage the reaction mixture is heated in a closed reaction vessel until a low molecular weight polyamide is formed; while in the second stage, this low molecular weight polyamide is converted to a high molecular weight polyamide upon additional heating.
Later, U.S. Pat. No. 3,847,876 to Onsager teaches another method for preparing a high molecular weight polyamide comprising contacting a dinitrile, a diamine and water. The Onsager process required at least equal molar amounts of diamine and dinitrile to be initially present in the reaction mixture and further requires that the polymerization occur in the presence of controlled amounts of ammonia, preferably in the presence of at least three weight percent based on the total weight of the diamine, dinitrile and water. This generally entails addition of ammonia to the reaction mixture over the course of the reaction.
More recently, Hoffmann et al., in U.S. Pat. No. 4,436,895, disclose an improved method for preparing a high molecular weight polyamide from a dinitrile, diamine and water by conducting the polymerization in the presence of one of (1) an oxygen-containing phosphorus compound, (2) an oxygen-containing boron compound (3) acidic oxygen-containing sulfur compounds and (4) hydrogen halide and ammonium or ammonium alkyl salts thereof. This process reduces the conventional reaction time of the polymerization and produces a high molecular weight, linear polyamide having desirable weight loss spinning properties.
Further, U.S. Pat. No. 4,520,190 to Coffey et al., describes the process for producing a polyamide by contacting a diamine, a dinitrile and water wherein the diamine is added gradually to the dinitrile over the course of polymerization in the presence of an oxygenated phosphorus compound catalyst.
A goal of these prior art processes was to produce nylon-6,6 suitable for fiber spinning, which could compete on a commercial basis with nylon-6,6 produced by the adipic acid/hexamethylenediamine route. While the prior art processes produce nylon-6,6, which can be made into nylon-6,6 fiber, the nylon-6,6 produced from dinitriles and diamines by these prior art processes is not ideal for fiber formation on a commercial scale. It has been discovered that the reason for this is the formation of impurities during the polymerization process. The impurities lower the crystallization rate of the polyamide and yield a polymer with poorer physical properties. The impurities also are subject to premature gelation, which accumulates and constricts manifold lines and fouls commercial spinning equipment after only a few hours of fiber production.
Specifically, the reaction of the dinitrile with a diamine in excess water to produce a polyamide can be shown as follows: ##STR1## It has been discovered that during the above polymerization reaction the diamines will react to form bis-(hexamethylene)triamine as follows: ##STR2## The --NH-- in the middle of the bis-(hexamethylene)triamine (hereinafter referred to simply as "triamine") provides a third reactive site for the triamine to react and attach and eventually branch and crosslink with polymeric chains. In order to produce commercial fiber grade quality nylon, the reaction of the dinitrile and diamine must yield a linear polymer. The branching and crosslinking of the triamine destroys the linearity of the polymer. Consequently, a process which limits the formation of triamine is desirable.
The objective of the instant invention is a process for the polymerization of dinitriles and diamines which will limit the formation of these triamine and other impurities and which will produce nylon-6,6 suitable for commercial fiber-forming operations.